Adjustable anti-backlash nut assembly

ABSTRACT

An anti-backlash nut assembly for translational movement along the axis of a leadscrew in response to rotational movement of the nut assembly relative to the leadscrew. The nut assembly comprises (i) a primary nut having an internal thread which mates with an external thread on the leadscrew; (ii) a secondary nut having an internal thread which mates with the external thread on the leadscrew, the secondary nut being capable of rotational movement about the leadscrew independent of the primary nut; (iii) a spring disposed intermediate the primary nut and the secondary nut; and (iv) a housing connected to the said primary nut which surrounds the spring and the secondary nut, for imparting rotational movement applied thereto simultaneously to the primary nut and the secondary nut. In a second embodiment, the nut assembly comprises (i) a primary nut having an internal thread which mates with an external thread on the leadscrew; (ii) a preload member having an internal thread which mates with the external thread on the leadscrew, the secondary nut being capable of rotational movement about the leadscrew independent of the primary nut; (iii) a spring disposed intermediate the preload member and an adjustment member; and (iv) an adjustment member connected to the housing and having a bore for insertion of the leadscrew therethrough; and (v) a housing connected to the primary nut which surrounds the spring, the preload member and the adjustment member, for imparting rotational movement applied thereto simultaneously to the primary nut and the preload member. The adjustment member capable of rotating about the leadscrew for adjusting the preload of the spring while the nut assembly is mounted on the leadscrew.

FIELD OF THE INVENTION

The present invention relates generally to motion control devices andmore particularly to anti-backlash nut assemblies designed fortranslational movement along a threaded leadscrew in response torotational movement of the nut relative to the leadscrew.

BACKGROUND OF THE INVENTION

In a variety of mechanical applications it is often required that oneelement of a machine be moved longitudinally with respect to anotherelement of the machine along a threaded leadscrew with repeatableaccuracy. One example of such an application is a print head which mustbe moved with respect to a platen on a data printer on repetitiveoccasions with precision.

One manner of providing such a motion control device is to attach amoveable machine element to a nut on the leadscrew. When the leadscrewis rotated, its rotational movement is translated to longitudinalmovement of the nut and the attached moveable element along thelongitudinal axis of the leadscrew. The position of the nut may beadjusted along the longitudinal axis of the leadscrew to determine therange of longitudinal movement of the element attached thereto.

Providing a nut which is adjustable with respect to the leadscrew,however, introduces the problem of backlash. Backlash is the play orslop which results from loose connections between mechanical elements,such as nuts and leadscrews. Because backlash introduces longitudinalpositioning errors, elimination of backlash in precision motion controldevices is desirable.

Mechanisms for reducing backlash in nut and leadscrew assemblies areknown. One manner of reducing backlash is to separate a nut on aleadscrew into two parts and install a compressed spring between the twoparts of the nut. The compressed spring applies a biasing force, whichis exerted in opposite longitudinal directions along the axis of theleadscrew, to the surfaces of both parts of the nut to which it iscoupled. The biasing force urges the internal threads on both portionsof the nut into contact with external threads on the leadscrew tothereby reduce backlash. Many prior art anti-backlash nut assembliesdisclose two-piece nut portions which are separated by a spring to forcethe internal threads on at least one of the two pieces of the nut intocontact with external threads on the leadscrew to reduce backlash.

One disadvantage of prior art anti-backlash nut assemblies is theygenerally do not have means for adjusting the compressive force orpreload provided by the spring intermediate the two portions of the nut,and especially when the nut assembly is installed on the leadscrew. Thecompressive force provided by the spring must be adjustable toaccommodate light or heavy tension applications. Moreover, adjustabilityof the compressive force of the spring without removing the nut assemblyfrom the leadscrew is important if it is desired to maintain the exactposition of the nut assembly on the leadscrew for continued operation.

Another disadvantage of prior art anti-backlash nut assemblies is thatthey generally have an externally mounted spring which results in ashortened life of the assembly due to exposure of the spring and theretaining mechanism to dirt or corrosive contaminants, as well ascontributing to a loss of spring lubricant. In addition, due to theexternal mounting of the spring, a mounting flange or other means mustbe utilized to mount any external assemblies such as a print head of aprinter. Also, the external spring mount design requires a largeroverall size of the nut assembly. Further, the exposed spring may be a"pinch point" or safety issue. Still further yet, prior art externalspring mount designs do not capture broken pieces should the springfail, which could result in serious damage to external machinery. Last,prior art external spring mount designs may result in a change ofpreload during load reversals.

Accordingly, it is highly desired to provide an improved anti-backlashnut and spring assembly having adjustment means to adjust the springpreload while the assembly is installed on a leadscrew while preventinga change in the amount of preload during load reversals. It is alsodesired to provide such a nut and spring assembly wherein the spring isinternally mounted within a housing to avoid exposure to contaminants,retain necessary lubricants, and provide an added safety feature wherebyextraneous matter may be caught in the spring. It is also desired toprovide an anti-backlash nut assembly with a reduced number of partswherein additional mounting flanges or other mounting parts are notneeded. Still further yet, it is desired to provide a design whichcaptures broken pieces should the spring fail, and thus preventingserious damage to external machinery.

SUMMARY OF THE INVENTION

The invention provides in one aspect an anti-backlash nut assemblydesigned for translational movement along the axis of a leadscrew inresponse to rotational movement of the nut assembly relative to theleadscrew. The anti-backlash nut assembly comprises a housing comprisinga primary nut therein, the primary nut having an internal thread whichmates with an external thread on the leadscrew. A secondary nut isdisposed within the housing and has an internal thread which mates withthe external thread on the leadscrew. The primary nut is capable ofrotational movement about the leadscrew independent of the secondarynut. A spring is contained within the housing and is disposed betweenthe primary nut and the secondary nut. The housing imparts rotationalmovement applied thereto simultaneously to the primary nut and thesecondary nut.

The invention provides in another aspect an adjustable anti-backlash nutand leadscrew assembly, comprising a housing connected to a primary nutwith an internal thread that mates with an external thread on theleadscrew. A preload member is disposed within the housing and has aninternal thread which mates with the external thread on the leadscrew.The primary nut is capable of rotational movement about the leadscrewindependent of the preload member. An adjustment member is connected tothe housing and has a hole for insertion of the leadscrew therethroughsuch that the adjustment member may be rotated over the leadscrew duringoperation for adjusting the spring preload. The spring is containedwithin the housing and is disposed between the preload member and theadjustment member for urging the primary nut and preload membertogether. The housing imparts rotational movement applied theretosimultaneously to the primary nut and the preload member.

The invention provides in yet another aspect an anti-backlash nutassembly designed for translational movement along the axis of aleadscrew in response to rotational movement of the nut assemblyrelative to the leadscrew, the assembly comprising a housing having aprimary nut contained therein, the primary nut having an internal threadwhich mates with an external thread on the leadscrew. A secondary nut isalso disposed within the housing, and has an internal thread which mateswith the external thread on the leadscrew. The primary nut is capable ofrotational movement about the leadscrew independent of the secondarynut. A spring is contained within the housing for preventing backlash ofthe primary and secondary nut upon the leadscrew and the housingimparting rotational movement applied thereto simultaneously to theprimary nut and the secondary nut.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a leadscrew and anti-backlash nutassembly constructed according to the principles of the presentinvention;

FIG. 2 is an exploded perspective view of the leadscrew andanti-backlash nut assembly as shown in FIG. 1:

FIG. 3 is a cross-sectional side view of the leadscrew and anti-backlashnut assembly as shown in FIG. 1;

FIG. 4 is a cross-sectional view of the lead screw and nut assembly asshown in FIG. 3 in the direction 4--4;

FIG. 5 is a cross-sectional view of the lead screw and nut assemblyshown with an alternative embodiment of the secondary nut as shown inFIG. 3 in the direction 4--4:

FIG. 6 is a perspective view of another leadscrew and anti-backlash nutassembly constructed according to the principles of the presentinvention;

FIG. 7 is a cross-sectional side view of the leadscrew and anti-backlashnut assembly as shown in FIG. 6;

FIG. 8 is a perspective view of the preload nut of the leadscrew andanti-backlash nut assembly as shown in FIG. 6;

FIG. 9 is a perspective view of the adjustment member of the leadscrewand anti-backlash nut assembly as shown in FIG. 6;

FIG. 10 is a cut-away view of the leadscrew and anti-backlash nutassembly as shown in FIG. 7 in the direction 10--10; and

FIG. 11 is an end view of the leadscrew and anti-backlash nut assemblyas shown in FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A first embodiment of a leadscrew and anti-backlash nut assembly 10constructed according to the principles of the present invention isshown in FIGS. 1 through 5. The anti-backlash nut assembly 10 comprisesa nut and spring arrangement 14 for operation on a leadscrew 12. The nutand spring arrangement 14 comprises a primary nut 16 a secondary nut 18,and a spring 20 disposed intermediate the primary 16 and secondary 18nuts. The primary nut 16 comprises an integral load bearing housing 22which surrounds the secondary nut 18 and the spring 20. The exteriorsurface of the integral housing 22 may comprise any desirable shape suchas a square, rectangle or any other suitable shape for mounting anycustom designed application. Alternatively, an optional flange ormachine element may be mounted on an external thread 30 located on theexterior surface of the primary nut 16 to provide an additional mountingsurface for a machine element which is desired to be movedlongitudinally along the axis 23 of the leadscrew 12 (not shown).

The leadscrew 12 is preferably comprised of steel, bronze or athermoplastic and is machined by conventional methods to provide anexternal thread 24 which runs substantially along its entire length. Theprimary nut 16 and the secondary nut 18 may also be comprised of anysuitable material such as aluminum, plastic, steel, or bronze. It ispreferred that the plastic have good machinability and lubrositycharacteristics such as acetyl plastic and acetron.

Both the primary nut 16 and the secondary nut 18 are provided with holeshaving internal threads 26 and 28, respectively, which corresponds tothe external thread 24 on the leadscrew. Rotational movement of theleadscrew 12 is translated into longitudinal movement of the nut andspring assembly 14 along a longitudinal axis 23 of the leadscrew 12.

As shown in FIG. 3, the interior of the integral housing 22 comprises afirst threaded portion comprising the primary nut 16, a second portion19 for receiving a spring 20, and a third portion 21 for receiving thesecondary nut 18. The three interior portions arc coaxial and eachportion preferably has a constant interior diameter. A first shoulder 32is formed between the interface of the second portion 19 and the firstportion 16 in which the first end of spring 20 is seated. The spring 20is disposed between the housing shoulder 32 and the inner member 38 ofthe secondary nut 18.

The secondary nut 18 thus comprises an outer member 36 and an innermember 38, forming a shoulder 37 therebetween. As previously discussed,the secondary nut 18 has an internally threaded hole 28 corresponding tothe threads 24 of a leadscrew 12. The secondary nut 18 is adapted forinsertion into the housing 22, with the outer member 36 beingpositionable within the third portion 21 of the housing, and the innermember 38 being positionable in either the third portion 21 or thesecond portion 19, depending upon the desired preload of the spring 20.The outer member 36 has a scalloped edge or grooves 40 on the outerperiphery. Alternatively, the scalloped edge or grooves 40 may compriseother shapes such as a triangle, star, hexagon, pentagon, or any othermulti-faceted configuration. For example, see the edge 40 shown in FIG.5. The scalloped edge 40 is adapted to be inserted into complementarymating surfaces or female internal grooves 42 located on the interiorsurface of the third portion 21, which mate with the scalloped edge 40.The inner member 38 of the secondary nut 18 has a smaller diameter thanthe diameter of the second portion 19 of the housing 22, and may beinserted into the second portion 19 until the outer member 36 engagesthe shoulder 34 formed between the interface of the second portion 19and the third portion 21. The shoulder 34 may thus serve as a mechanicalstop to prevent further compression of the spring 20 by the secondarynut 18.

The spring 20, when mounted in this manner, applies a biasing force inopposite longitudinal directions along the axis 23 of the leadscrew 12and thus urges the primary 16 and secondary 18 nuts in oppositedirections, thereby inhibiting backlash. Minimization of backlash isimportant in close tolerance positioning applications. The amount offorce applied to each of the primary and secondary nuts 16, 18 by thecompressed spring 20 to minimize backlash depends on the preloadedcompression of the spring 20. Variance in the compression of the spring20 is accomplished by unscrewing the secondary nut 18 completely fromthe leadscrew 12, rotating the secondary nut 18 clockwise to increasethe preload while maintaining contact with the spring 20, and thenthreading the secondary nut 18 onto the leadscrew 12 when the desiredpreload is set. The scalloped edges 40 of the secondary nut 18 allow forvery fine adjustment of the preload of the spring. Thus, rotating thesecondary nut 18 while in contact with the spring 20 increases ordecreases the length of the spring, and hence the distance between theprimary nut 16 and secondary nut 18. For coarser adjustment of thespring 20 preload, the secondary nut 18 may be slid inward or outward inthe grooves 42 of the third portion 21 of the housing 22 for the desiredamount of spring preload when the secondary nut 18 is unscrewed from theleadscrew 12.

The preloaded value of spring 20 is thus finely adjustable between afirst position wherein the shoulders 34,37 of the primary and secondarynuts abut (maximum available compressive force for a given spring) to asecond position wherein the spring is almost totally relaxed (minimumcompressive force). The compressive force exerted upon the primary 16and secondary 18 nuts may also be altered by changing the size of thespring 20 to thereby change the spring constant.

A second embodiment of a leadscrew and anti-backlash nut assembly 50 asshown in FIGS. 6 through 11 may now be described. The anti-backlash nutassembly 50 comprises a nut and spring arrangement 52 for operation on aleadscrew 12. The nut and spring arrangement 52 comprises a primary nut16 integral with a load bearing housing 22 a preload nut 60, anadjustment member 70 and a spring 20 disposed within the housing 22.

The primary nut 16 is preferably integrally molded or machined with aload bearing housing 22 which contains the preload nut 60, theadjustment member 70 and the spring 20. The exterior surface of theintegral housing 22 may comprise any desirable shape such as a square,rectangle or any other desired shape for mounting any custom designedapplication. Alternatively, a flange or machine element may be mountedon the external thread 30 located on the exterior surface of the primarynut 16 to provide an additional mounting surface for a machine elementwhich is desired to be moved longitudinally along the axis 23 of theleadscrew 12.

Both the primary nut 16 and the preload member 60 are provided withinternally threaded inner diameters 26 and 62, respectively, whichcorresponds to the external thread 24 on the leadscrew 12. Rotationalmovement of the leadscrew 12 is translated into longitudinal movement ofthe nut and spring assembly 50 along a longitudinal axis 23 of theleadscrew 12.

The interior of the integral housing 22 comprises a first threadedsection 63 containing an integral primary nut 16, a second section 64for receiving the preload member 60, a third section 66 for receiving aspring 20, and a fourth section 68 for receiving the adjustment member70. The four interior sections of the housing 22 are coaxially alignedabout axis 23 and each section except the second section 64, preferablyhas a constant interior diameter.

The cross-sectional shape 67 of the preload member 60 and the femalemating second section 64 of housing 22 may comprise any shape such as atriangle, rectangle, or any other shape which prevents rotation of thepreload member 60 with respect to the housing 24 while being threadedupon the leadscrew 12 and during operation. Preload member 60 preferablyhas a square cross-sectional shape 67 with rounded corners which isinserted into the mating second section 64 of housing 22. The outer lip72 of the preload member 60 bears against shoulder 74 formed between theinterface of sections 64 and 66, when the spring 20 is compressed orpreloaded at the upper limit of the preload. A gap 76 exists between thepreload member 60 and the primary nut 16 during operation of theassembly, such that the each member independently rotates about theleadscrew 12. As the compression of the spring 20 is reduced, preloadmember 60 is free to axially move within the second section 64, therebyincreasing the gap 76.

The spring 20 is disposed in the third section 66 of the housing 22between the preload member 60 and the shoulder 78 of the adjustmentmember 70. The maximum available amount of spring 20 preload isdetermined by the axial length of the spring 20 and the spring constantof the spring 20. Adjustment member 70 may be adjusted to compress thespring 20 while installed upon the leadscrew 12 until the shoulder 78contacts the stop 82 formed at the intersection of section three 66 andsection four 68. This unique adjustment feature may occur because theadjustment member 70 is not threaded upon the leadscrew 12, and instead,has a smooth inner diameter surface 83 larger than the diameter of theleadscrew 12. Adjustment member 70 is thus mounted to the interiorhousing 22 by an external thread 80 which mates with the interiorthreaded surface of section four 68. The external face 84 of theadjustment member 70 contains one or more holes 86 for the insertion ofa spanner wrench or even the end of a paper clip, to aid in theadjustment of the spring 20 preload, as described further, below.

The spring 20, when mounted in this manner, applies a biasing, force inopposite longitudinal directions along the axis 23 of the leadscrew 12and thus urges the primary nut 16 and adjustment member 70 in oppositedirections. The adjustment member 70 transmits the spring force to thehousing 22, resulting in the primary nut 16 being urged towards theadjustment member 70 to thereby inhibiting backlash. Thus while priorart anti-backlash assemblies generally eliminate backlash by urging theprimary nut 16 and preload member 60 away from each other using aspring, backlash may also be eliminated by forcing the primary nut andpreload member towards each other. The unique design of the inventionenables backlash to be essentially eliminated while allowing for preloadadjustment while the assembly is mounted upon the leadscrew 12. This isa unique feature not found in the prior art.

The amount of force applied to the primary nut 16 and preload member 60by the compressed spring 20 to minimize backlash depends on thepreloaded compression of the spring 20. Variance in the compression ofthe spring 20 is easily accomplished while the assembly 52 is installedupon the leadscrew 12, with the utilization of a spanner wrench, snapring pliers, or even a paper clip end. In order to adjust the preload, aspanner wrench may be inserted into one of the holes 86 located on theface 84 of the adjustment member 70. While the housing 92 is heldstationary upon the linearscrew 12, the wrench or paper clip is insertedinto holes 86 and is used to manually rotate the adjustment member 70clockwise to decrease the preload and counterclockwise to increase thespring preload. (These directional references refer to a right-handedlinear screw, and would be opposite for a left-handed screw.) Thepreload value of spring 20 is thus finely adjustable between a firstposition wherein the shoulder 78 of the adjustment member 70 intersectsthe stop 82 of the housing 22 (maximum available compressive force for agiven spring) to a second position wherein the spring is almost totallyrelaxed (minimum compressive force). The compressive force exerted bythe spring 20 may be altered by changing the size of the spring tothereby change the spring constant.

Thus, this embodiment of the invention permits adjustability of thecompression of an internally contained spring of a anti-backlash nut andspring assembly while the assembly is installed on the leadscrew. Inaddition, because of the unique design of the housing 22, the problem ofan undesired change in spring preload during load reversal isessentially eliminated. Accordingly, the preferred embodiment of aanti-backlash nut and leadscrew assembly has been described. With theforegoing description in mind, however, it is understood that thisdescription is made only by way of example, that the invention is notlimited to the particular embodiments described herein, and that variousrearrangements, modifications and substitutions may be implementedwithout departing from the true spirit of the invention as hereinafterclaimed.

What is claimed is:
 1. An adjustable anti-backlash nut and leadscrewassembly, comprising:a housing comprising a primary nut connectedthereto, said primary nut having an internal thread which mates with anexternal thread on the leadscrew. a preload member disposed within saidhousing, said preload member having an internal thread which mates withthe external thread on the leadscrew, said preload member having meansfor preventing relative rotational movement between said preload memberand said housing; said primary nut being capable of rotational movementabout the leadscrew independent of said preload member; an adjustmentmember threadedly connected to said housing and having a hole forinsertion of the leadscrew therethrough whereby the adjustment membermay be rotated about the leadscrew during operation for adjusting thepreload; a spring contained within said housing and disposed betweensaid preload member and said adjustment member for urging said primarynut and preload member towards each other; and said housing impartingrotational movement applied thereto simultaneously to said primary nutand said preload member.
 2. The nut assembly of claim 1, wherein saidprimary nut is integrally connected to said housing.
 3. The nut assemblyof claim 1, wherein said preload member comprises one or more grooves,and an interior surface of said housing is provided with a complementarymating surface for preventing rotational movement of the preload memberwith respect to the housing.
 4. The nut assembly of claim 1, whereinsaid adjustment member comprises axial holes for insertion of means forpreventing rotation of said adjustment member while said housing isrotated.
 5. The nut assembly of claim 1, wherein a gap is providedbetween said primary nut and said preload member.
 6. The nut assembly ofclaim 1, wherein said periphery of said preload member has one or moregrooves, and said interior surface of said housing is provided withcomplementary-mating surface.
 7. The nut assembly of claim 1, whereinsaid primary nut and said secondary nut are comprised of metal.
 8. Thenut assembly of claim 1, wherein the interior surface of said housingcomprises a means for adjusting the compression of said spring.
 9. Thenut assembly of claim 1, wherein said housing comprises external threadsfor the mounting of a flange.
 10. The nut assembly of claim 1, whereinsaid periphery of said preload member is square shaped, and saidinterior surface of said housing is provided with a complementary-matingsurface.
 11. An anti-backlash nut assembly for translational movementalong the axis of a leadscrew in response to rotational movement of thenut assembly relative to the leadscrew, comprising:a housing comprisinga primary nut therein, said primary nut having an internal thread whichmates with an external thread on the leadscrew; a secondary nut disposedwithin said housing, said secondary nut having an internal thread whichmates with the external thread on the leadscrew, said primary nut beingcapable of rotational movement about the leadscrew independent of saidsecondary nut; a spring contained within said housing for preventingbacklash of said primary and secondary nut upon said leadscrew; and saidhousing imparting rotational movement applied thereto simultaneously tosaid primary nut and said secondary nut.
 12. The assembly of claim 11wherein said primary nut is integrally connected with said housing. 13.The assembly of claim 11 wherein said spring is disposed between saidprimary and secondary nut.
 14. The assembly of claim 11 furthercomprising:an adjustment member threadedly connected to said housing andhaving a hole for insertion of said lead screw; said spring beingdisposed between said primary nut and said adjustment member wherebysaid spring urges said primary nut towards said secondary nut; and anadjustment member being capable of adjusting the spring preload duringoperation of the assembly.
 15. The assembly of claim 14 wherein aspatial gap is formed between a housing shoulder and said secondarymember.